Meadowlark reports:
Grass carp have their pros and cons...one of the biggest cons is I can't keep them in the pond. In fall and winter here we sometimes get tremendous rains of many inches at a time which causes a significant outflow of water over the spillways
Maybe that's one disadvantage of natural spillways. I think it is easier to fence/strain the input to a drainpipe than an aboveground spillway.
Or maybe, Meadowlark, you just get some hellaciously big Frog-Strangler rains.